This invention relates to linear stepper motors. In particular, the invention is concerned with a system for forming components of the linear stepper motor, and for enhancing the operational performance of such motors.
Linear stepper motors are used for positioning applications requiring rapid acceleration and high speed moves with low mass payloads. Mechanical simplicity and precise open loop operation are additional features of the stepper linear motor systems.
A linear stepping motor operates on the same electromagnetic principles as a rotary stepping motor. The stationary part or platen is a passive toothed steel bar extending over the desired length of travel. Permanent magnets, electromagnets with teeth, and bearings are incorporated into the moving elements or forcer. The forcer moves bidirectionally along the platen, assuring discrete locations in response to the state of the currents in the field windings. In general, the motor is two-phase, however a larger number of phases can be employed.
A linear stepper motor is not subject to the same linear velocity and acceleration limitations inherent in systems converting rotary to linear motion. With linear motors, the force generated by the motor is efficiently applied directly to the load and length has no effect on system inertia. Additional benefits of linear stepper motors include:
1. The motors are capable of speeds to 100 ips and the low mass forcer allows high acceleration.
2. The need for lead screws or belts and pulleys is eliminated.
3. Length of travel is limited by the length of the platen, and increasing the platen length causes no degradation in performance.
4. Precise open loop operation is possible, and unidirectional repeatability to 2.5 micron is possible without the added expenses of feedback devices.
5. A linear motor is usually smaller in all three dimensions than comparable systems where rotary motion is converted to linear.
6. More than one forcer can operate on the same platen with overlapping trajectories.
A critical requirement to ensure the accuracy of stepper motors is to have accurate positioning of the teeth in the platen and the teeth in the forcer. Conventionally, it is possible to form the teeth in a platen by an etching process, and the nature of etching is such that these teeth can be accurately positioned. This has not been possible, to accurately form the teeth in the pole faces in the electromagnets forming part of the forcer. Accordingly, a limitation exists in the accuracy of linear stepper motors due to this characteristic.
There is accordingly a need to provide a stepper motor with enhanced ability to accurately position the teeth in the forcer. Moreover, it is desirable to provide systems to increase the magnetic flux density of both the platen and the teeth in the forcer.